It is known to provide reliable storage and retrieval of digital data, particularly computer data, by means of a digital data storage (DDS) format defined in ISO/IEC standard 10777:1991E. DDS format devices have developed through versions DDS-1 to DDS-4, which are known in the prior art.
In a DDS device, an elongated band of magnetic tape contained upon a pair of spools in a data storage cassette is transported past one or more rotating electro-magnetic read and write heads, such that the heads trace a path which is substantially diagonal across a main length of the elongated magnetic tape. Multiple passes of the write heads result in multiple diagonal tracks across the magnetic tape, which extend along the length of the magnetic tape.
Referring to FIG. 1 herein, there is shown schematically a layout of a tape data storage cartridge in relation to a tape drive mechanism according to the DDS format, in which an elongated band of tape is contained within a removable tape cartridge 100. The tape cartridge is inserted into the tape drive mechanism. A rotating read/write head 101 comprises first and second read heads and first and second write heads situated at substantially equidistant points around a circumference of the rotating head. The head rotates on top of a substantially cylindrical metallic plinth 102. The read/write head rotates at a speed of approximately 11,400 revolutions per minute. A main central axis of a cylinder formed by the outer surfaces of the drum and the plinth is directed offset from a line normal to a plane of a base plate 103, so that the effect is that as the band of tape traverses around part of the circumference of the cylindrical head plinth, the rotating heads describe a path diagonally across the width of the tape in successive passes of the heads past the tape.
Referring to FIG. 2 herein there is shown schematically a tape path of the elongated magnetic tape data storage medium 201 as it is drawn past the rotating drum containing the read and write heads. The tape data storage medium 201 is wound onto a feed reel 202 and a take up reel 203 which are within the removable tape cartridge 100. During normal operation, the magnetic tape 201 is wound from the feed-reel 202 onto the take-up reel 203. The path of the magnetic tape 201 is constrained by a plurality of rollers and tape guides 204-208. Additional tape guides 104,105 determine the relative positions of the rotating drum 102, the read and write heads 210-213 and the tape data storage medium 201. The feed reel 202 and take up reel 203 are driven by electric motors to maintain a correct tension in the magnetic tape 201 past the head.
Referring to FIG. 3 herein, there is illustrated schematically the orientation of the magnetic tape 201 with respect to the rotating drum 101. The tape 201 is drawn past the rotating head at a relatively slow tape speed of the order of a few centimeters per second. However, the rotating drum 101 on which the read and write heads are mounted typically rotates at a few thousand revolutions per minute, so the relative speed of the read and write heads to the drum is order of magnitudes greater than the absolute tape speed. During a write operation, the write heads record a sequence of tracks diagonally across the elongated magnetic tape 201. The width of such tracks is typically of the order of 6.8 μm.
Referring to FIG. 4 herein, there is illustrated schematically a portion of a write circuit for writing a logical track according to the specific implementation of the present invention. The write circuit contains a linear feedback shift register 400 for generating a pseudo random bit sequence as described herein for incorporation into a preamble field of the logical data track; an 8-10 encoder 401; a non-return to zero circuit 402, an output amplifier 403, and a write head 404.
Referring to FIG. 5 herein, there is shown schematically a read channel for reading data from a data storage medium of cartridge 100. The read channel comprises a read head 500. Data stored on the tape is read by the read head 500 which passes the signal via a rotary transformer 501 to an amplifier 502. Amplifier 502 sends an amplified output signal which is supplied to an equalizer 503 for the purpose of initial equalization. After equalization, the signal is passed to an automatic gain control circuit 504, and is filtered in a filter 505 which further shapes an overall channel frequency response to match a required equalization characteristic. The filtered signal is supplied to an analog to digital converter 506 which produces a digitized version of the filtered signal, which is then passed to a feed forward equalizer 507 which further equalizes the signal to a required equalization target. An equalized digital signal output from the feed forward equalizer 507 is supplied to a sequence detector 508. The sequence detector 508 includes a Viterbi engine, and various detection paths for determining a sequence of bits resulting from the signal read by the read head 500. The read channel also includes a preamble detector 509 for detecting preamble data before reading user data, the preamble detector producing an output which is supplied to a state machine 510. The output of the state machine controls automatic gain control circuit 504 to adjust a gain in the read channel.
Referring to FIG. 6 herein, there is illustrated schematically a layout of physical tracks striped across the width of an elongated magnetic band tape in a cartridge. A plurality of tracks are written slightly overlaying each other by successive passes of the write head of a rotating drum.
Referring to FIG. 7 herein, there is illustrated schematically a logical data layout of a single track written across the tape data storage medium in a single pass of a write head across the width of the tape. The logical track 700 comprises in sequence a first margin area 701 which when written physically resides at one edge of the tape data storage medium; a preamble region 702; a user data region 703 that is close to region 702, and preceded by a synchronization header 704; and a second margin area 705 written after the user data 703, the second margin area laying physically at a second edge of the tape data storage medium. The preamble region 702 is positioned between the first margin area 701 and the user data 703 so region 702 immediately precedes the user data 703. The preamble region acquires gain and timing information prior to reading the user data 703 with the object of achieving an optimum bit error rate while reading the user data area 703. Because the head reads preamble region 702 immediately before user data region 703, an AGC circuit driven by the head does not change gain substantially when there is a transition from region 702 to region 703.
The prior art preamble field contains a 2T tone data. However, as the density of bits stored on tape increases, the 2T tone data preamble field gives rise to a problem of fluctuation of gain control in the automatic gain control circuit 504 which causes problems for reading higher bit densities.
In the prior art DDS-4 format, the preamble region 702 usually consists of a single frequency constant tone bit sequence which immediately precedes the user data 703. The DDS-4 logical format calls for a constant 2T tone in the preamble region. The parameter T relates to the minimum acceptable spacing between pulse transitions. In the DDS-4 format the preamble region 702 consists of 640 bits of data. The bits of data are arranged in transitions of 2T length, that is to say 4 bits per cycle. This gives a preamble region length of 160 cycles (640T), each cycle being +,+,−,−. The physical distance occupied by a cycle on the tape data storage medium depends on the data storage density of bits on the tape. The overall physical length of preamble field as recorded on the tape in the DDS-4 format is approximately 107 μm.
According to one aspect of the co-pending application there is provided a method of encoding a plurality of tracks of data for storage on a tape data storage medium. The method comprises for each of said plurality of data tracks written to said tape data storage medium, writing within said data track, a preamble region which precedes a user data region, said preamble region having a preamble data sequence having a power spectrum substantially similar to a power spectrum of a substantially random data. Preferably said preamble data sequence of the co-pending application has a power spectrum characteristic prior to any subsequent encoding which may occur has a power spectrum resulting from the following byte stream of hexadecimal coded numbers b3, d2, b8, 83, 67, a5, 71, 06, cf, 4a, e2, 0d, 9e, 95, c4, 1b, 3d, 2b, 88, 36, 7a, 57, 10, 6c, f4, ae, 20, d9, e9, 5c, 41, b3, d2, b8, 83, 67, a5, 71, 06, cf, 4a, e2, 0d, 9e, 95, c4, 1b, 3d, 2b, 88, 36, 7a, 57, 10, 6c, f4, ae, 20, d9, e9, 5c, 41, b3, d2, b8, 83, 67, a5, 71, 06, cf, 4a, e2, 0d, 9se, 95, c4, 1b, 3d, 2b, 88, 36, 7a, 57.
According to another aspect of the co-pending application there is provided an apparatus for encoding a plurality of tracks of data for storage on a tape data storage medium, said apparatus comprising:
a write circuit for writing said plurality of data tracks to a tape data storage medium, said write circuit being arranged for writing on said data track, a preamble region which precedes a user data region, said preamble region having a preamble data sequence having a power spectrum substantially similar to a power spectrum of a substantially random data.
In the co-pending application the apparatus writes a preamble data sequence having a power spectrum characteristic resulting from the following byte stream of hexadecimal numbers:
b3, d2, b8, 83, 67, a5, 71, 06, cf, 4a, e2, 0d, 9e, 95, c4, 1b, 3d, 2b, 88, 36, 7a, 57, 10, 6c, f4, ae, 20, d9, e9, 5c, 41, b3, d2, b8, 83, 67, a5, 71, 06, cf, 4a, e2, 0d, 9e, 95, c4, 1b, 3d, 2b, 88, 36, 7a, 57, 10, 6c, f4, ae, 20, d9, e9, 5c, 41, b3, d2, b8, 83, 67, a5, 71, 06, cf, 4a, e2, 0d, 9e, 95, c4, 1b, 3d, 2b, 88, 36, 7a, 57.